Many data communications systems include a master controller operating to control the operation of a plurality of remotely located devices. In controlling such remotely located devices, the master controller may both transmit and receive data from each one of the devices. For example, such a data communications system may be located in a modern gasoline service station, where a master controller located in the office of the station is connected to both transmit and receive data from current loop transceivers located in a plurality of pumps or fuel dispensing stations, respectively. In such a typical data communications system, where a plurality of current loop transceivers, for example, are communicating with a single master controller or similar device, if one of the current loop transceivers fails by either open-circuiting its connection to the data communication line, or shorting to ground the data communication line, the master controller loses communication with all of the remote devices. To restore communication therebetween, the defective device or current loop transceiver must be isolated from the communication data path. The typical method for isolating such malfunctioning devices or current loops, for example, is to physically switch the failed loop out of the system by manually switching each one of the remote units into and out of the data communication path, until the defective remote device is located. Such a method can be very time consuming, and may result in considerable data being lost between the master controller and the operational ones of the remote devices.
In the prior art, a number of circuits and methods have been developed for isolating a malfunctioning device in a data communication system. For example, in Hono et al. U.S. Pat. No. 4,685,113, a transceiver for a common bus-type transmission system of a local area network includes a receiver 3 for receiving data from a bus 2, and a switching transistor 6 for transmitting data signals from the transceiver to the bus 2, with the data transmission circuit also including a diode 7 between the collector of the transistor 6 and bus 2, with the diode 7 being polarized for blocking received data signals flowing from the bus 2 to the transistor 6, and further includes a first data detection circuit at the base of the transistor 6 for detecting data signals for transmission, a second data detection circuit coupled to the collector of the transistor 6 for sensing the presence of data signals to be transmitted to the bus 2, a determination circuit 11 having first and second input terminals connected to the output terminals of the first and second data detection circuits 9 and 10, respectively, for determining whether data transmission signals are substantially simultaneously present at both the base and collector electrodes of the transistor 6, for closing a switch 12 upon such simultaneous occurrence for permitting data signals to be coupled through the diode 7 to the bus 2, or for opening the switch 12 to prevent such transmission of data signals during times that such signals are not detected simultaneously by both the first and second data detection circuits. In this manner, if either the transistor 6 or diode 7 are defective, the data transmission driver circuit 5 of the transceiver is disconnected from the bus 2, to prevent interference with data communication between the bus 2 and other transceivers connected to the bus. However, data signals from the bus 2 continue to be received by the transceiver via the receiver 3.
Georgopoulos, U.S. Pat. No. 3,974,403, discloses a digital data transmission system, including a common transmission line, a plurality of input/output devices coupled to the transmission line, with each such device including an output transistor coupled to a voltage supply through a first transistor, with the system further including an electronic switch coupled to the base electrode of the first transistor of each one of the plurality input/output devices for interrupting the flow of base current to the first transistor whenever a short circuit condition occurs in the driver circuit, for removing the driver circuit from the transmission line.
Adams, U.S. Pat. No. 3,716,834 discloses a system for use with a data transmission network having a central station and a plurality of remote stations connected in series by a low grade transmission line, such as a two conductor telephone line. The system includes detection circuitry for detecting transmission line faults caused by defects in one or more of the remote stations such as short circuits to ground, open circuit conditions, line-to-line shorts, and so forth, whereby the detection circuitry operates to open switches for decoupling the defective ones of the remote stations from the series line, while at the same time, closing other switches to bypass the defective remote stations to maintain the series connected transmission network between the central station and the operating ones of the remote stations.
Polkinghorn et al., U.S. Pat. No. 3,579,275 discloses an isolation circuit for gating devices for charging and discharging a number of capacitors during different recurring time intervals, for turning on an MOS device as a function of the charge on a particular one of the capacitors serving as a feedback capacitor, for driving the output of the gating device to approximately the voltage level of a clock source, for neutralizing noise appearing at the output and for providing a low output impedance.